1. Technical Field
The present invention relates, generally, to fail passive servo controllers and systems, and more particularly, to fail passive servo controllers for brushless DC motors.
2. Background Art and Technical Problems
Various aircraft control systems have significantly improved safety, precision, and efficiency of travel. Many of these improvements are due in part to the control systems automating tasks that were previously performed by the vehicle operator. Such systems typically comprise a control unit which communicates with various servo units which in turn control the actions of the aircraft (e.g., turn, roll, climb, etc.). The servos generally include a motor, a gear unit and a clutch for engaging the motor and gear. When the motor and gear engage, among other functions, the servo acts to activate/manipulate control cables which control the actions of the aircraft. For example, autopilot servos may be used to control the aircraft during various stages of a flight plan.
Similarly, automatic landing systems (ALS), are often used to aid in landing the aircraft. However, given that landing an aircraft requires extremely precise control, the ALS should be fail passive. Stated otherwise, should the ALS fail, the ALS should not cause the aircraft to act in an undesirable manner (such as a sudden turn, roll or dive). For example, a non-fail passive servo drive has the potential to, in the absence of a command to do so, cause the servo to drive the motor and move the control cable(s) in one direction or another. This is often referred to as a xe2x80x9crunawayxe2x80x9d condition. When a runaway condition occurs, either system monitors or the pilot must detect the failure and disconnect the system, placing the aircraft under manual control.
Various landing standards are defined in the industry for describing the required precision of landing approaches. Exemplary standards include Category 1, or approximately one half mile visibility; Category 2, one eighth to one quarter mile visibility; and Category 3, with as little as 700 feet of visibility. Depending on the weather, in extreme conditions, visibility may be so low that the pilot is unable to see the aircraft""s position relative to the runway, and the pilot must rely much more on the ALS system down to and including touchdown and roll out. Thus, at such times, precise ALSs may be used all the way through landing of the aircraft and is referred to as an xe2x80x9cautolandxe2x80x9d. In many instances, ALS failures are minor and no hazards are posed. However, as the precision for an autoland increases, even minor failures can become more hazardous.
Further, current ALS systems typically used brushed DC motors. Brushed motors typically have multiple sets of rotor windings encased by one or more magnets. When the rotor windings are energized by an electric current, the rotor is polarized and the motor switches from one set of windings to the next. As the motor commutates, the rotor switches to the next set of windings, and a torque is created between the magnetic fields. However, brushed motor reliability is limited by brush wear and contaminant interaction with brush dust caused from normal use of the motor. In addition, brushed motors suffer performance disadvantages in the areas of thermal management and rotor inertia with respect to brushless motors.
Accordingly, methods, apparatus and systems are therefore needed in order to overcome these and other limitations of the prior art. Specifically, there is a need for application of fail passive DC motor control to brushless DC motors.
The present invention provides apparatus, systems and methods for fail passive servo controllers. In accordance with an exemplary embodiment of the present invention, servo controller is used with an ALS to control an aircraft during landing procedures. In accordance with one aspect of this exemplary embodiment, upon a failure (e.g., of one of the components of ALS) servo controller fails passively rather than actively. In an exemplary embodiment, the fail passive nature of the servo unit is accomplished by providing at least two independent signal and drive current paths for controlling the servo motor. As such, the servo motor will not generate torque unless the two independent signal lanes agree. For example, when a component such as processor, difference amplifier, or pulse width modulator (PWM) fails or otherwise ceases to function properly, servo motor will not runaway, but rather will fail with only minor transient movement, or alternatively, servo unit will lock at its current position. In this manner, servo motor will not generate a deviation from flight path or other significant control inputs until system monitors detect the fault and return control to the pilot.